I. Field
The following description relates generally to communications systems, and more particularly to performing accurate signal path determinations for mobile wireless communications systems.
II. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so forth. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can concurrently support communication for multiple wireless terminals that communicate with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-signal-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Generally, each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system also supports time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows estimation of the forward link channel from the reverse link channel. This enables an access point to extract transmit beam-forming gain on the forward link when multiple antennas are available at the access point.
Wireless systems transmit pilot signals or reference signals from base stations to receivers in order to facilitate communications. These and other signals are analyzed by receivers during specific time windows. For instance, a signal for a first base station may be analyzed during a first time period, a second base station during a second time period and so forth. As is the case with almost all mobile wireless devices however, in order to conserve power, the devices are often put into sleep mode, where only basic communications are maintained with the base stations. Every one hundred milliseconds or so, the devices will wake out of sleep mode and perform calculations such as a channel estimation. Due to the nature of how signals are analyzed according to various time windows, and given that the devices may only observe what has happened intermittently in order to conserve power, errors may be encountered when trying to analyze which signals have been received from the respective base stations. In other words, it is possible that one or more signals for a first base station could be captured and observed in a time window designated for another base station. This may cause inaccurate determinations at the receivers.
In general, there are several potential causes for signals to drift outside a given observation window. In one case, the velocity at which a mobile device is traveling can contribute to the problem. This is not a significant problem since mobile velocities are much slower that signal velocities that are at the speed of light. Another problem can include signals that have fairly long multi-path components that cause portions of the signal to appear improperly in a neighboring observation window. Perhaps the most insidious problem is drift caused by clock error rates in the receiver. If base station signals were continually being monitored, it would be possible to track and account for any resultant drifts. Unfortunately, since the receivers are in sleep mode for much of the time, it is not possible to track how signal paths have changed over time.